Histoincompatible marrow grafts following otherwise lethal total body irradiation (TBI) often fail, with the risk of graft failure directly proportional to the degree of histoincompatibility between donor and recipient. The mechanism of failure is attributed to destruction of the transplanted cells by residual immunocompetent cells of the host. Pretreatment of canine marrow recipients with monoclonal antibody (MAb) S5, before TBI and transplantation, facilitates engraftment of unrelated dog leukocyte antigen (DLA)-nonidentical marrow. Preliminary studies show that the molecule recognized by MAb S5 has structural homology to CD44, which is a leukocyte adhesion receptor that modulates immune function of CD2 and CD3 and induces cytokine secretion. To define the role of the adhesion molecule CD44 in graft rejection, we will evaluate the anti-CD44 effects on immune function and define the cells and epitopes recognized by the MAb. The effects of MAb S5 on immune function will be examined by both in vitro assays and in vivo studies including skin transplant and lymphocyte trafficking experiments. The epitope or structure of the CD44 molecule responsible for graft rejection will be defined by molecular analysis and peptide mapping and confirmed by in vitro and in vivo studies involving soluble CD44. Quantitative polymerase chain reaction and RNase protection assays for mRNA of specific cytokines, along with cytokine protein assays, will evaluate whether binding of CD44 has an effect on cytokine production, and thereby enhances engraftment indirectly. Using an autologous transplant model, the role of CD44 on stem cells will be examined both by anti-CD44 MAb administration and by CD44-positive marrow cell selection and transplant. In collaboration with project 0013, the effect of MAbs specific for CD44 on progenitor cell-stromal cell interactions will be determined in long-term cultures of human bone marrow. Understanding the mechanism by which an antibody specific for the CD44 adhesion receptor facilitates engraftment could have direct application to clinical marrow transplantation, particularly in regard to HLA-nonidentical transplants.